1. Field of the Invention
The present invention relates to an ferrodielectric liquid crystal display (FLCD) manufacturing method and, more particularly, to an FLCD manufacturing method capable of forming a ferrodielectric crystal in a uniform mono domain upon a bookshelf-structured crystalization to enhance image quality.
The present application is based on Korean Patent Application No. 2002-18086 filed on Apr. 2, 2002, which is incorporated herein by reference in its entirety.
2. Description of the Prior Art
The LCD is a flat-type display device use of which is wide spread. The LCD is used mainly for portable devices and is also replacing conventional cathode-ray tubes (CRTs) in the field of large-sized display devices with the developments of display-enlarging technologies.
There exist various kinds of crystal materials applied to form the LCD.
The twisted nematic (hereinafter referred to as “TN”) LCD employed a lot in general applications uses the interactions between the dielectric anisotropy of the TN liquid crystal molecules and the electric field to slow down a response time in tens of miliseconds (ms) so that it has difficulties in displaying animations, has a narrow visual angle, and generates cross-talk among pixels within a certain distance, to thereby cause a problem of difficulties in reducing a pixel size below a certain size.
Meanwhile, the FLCD uses the interactions between spontaneous polarization of the ferrodielectric liquid crystal and the electric field and provides rapid response characteristics below 1 ms. Such an FLCD, as a next generation display device, has no difficulties in displaying animations, is capable of providing a wide visual angle, and can implement a high resolution with a more reduced pixel size preventing cross-talk from occurring due to strong interactions between molecules, so that research into the FLCD is actively progressing.
For the ferrodielectric liquid crystal widely employed at present, there is a chiral smectic C phase (SmC*) liquid crystal material having bistable characteristics and a chevron structure.
For the FLCD using the liquid crystal material, in the manufacturing process, crystal maintained at a temperature over the melting point is injected in a cell between the substrates, the crystal turns to its smectic A phase having a layer structure perpendicular to the rubbing direction via a chiral nematic (N*) phase when the temperature is lowered down, and then turns to the chiral smectic C phase. In the process, the molecular long axes of liquid crystal molecules in the liquid crystal layer is tilted at a particular angle to the rubbing direction to decrease a distance between smectic layers, as a result of which the bent of the smectic layer is induced in the liquid crystal layer to compensate a volume change. Such a bent layer structure is referred to as a chevron structure, and a domain having different long axis directions of the liquid crystal based on the bent direction is formed, and on the interface of which non-uniform orientations having zigzag defects, hairpin defects, mountain defects, and so on are obtained.
A contrast ratio of an LCD is remarkably deteriorated with such orientation characteristics, and, if a dc voltage is forced to be applied against it, ions in the liquid crystal layer are stacked up or absorbed on the surface of an orientation film to cause a problem that produces a residual image phenomenon faintly displaying a preceding image pattern as one display is changed to the next one.
Besides, research is actively being carried out on ferrodielectric liquid crystal materials which provide an anti-ferrodielectric liquid crystal (AFLC) mode alleviating a threshold limitation, but its spontaneous polarization is more than 100 nC/cm2 so that a residual image may also occur due to ion movements caused by an inverse polarization electric field. Further, in case of applying the active matrix driving approach which is an approach for each pixel to independently drive a liquid crystal by using thin film transistors (TFTs), leakage currents may occur owing to big spontaneous polarizations. Capacitors should be very large in capacity in order to restrain the leakage current of the AFLC, which reduces an aperture ratio to cause a problem of difficulties in using the AFLC as a display device.
In order to improve the drawback to such a ferrodielectric liquid crystal, ferrodielectric liquid substances have been continuously developed which have a bookshelf structure enabling alternate current driving and restraining residual images.
There is a crystal substance, as a ferrodielectric liquid crystal substance having the bookshelf structure in the spotlight at present, which has skipped the smectic A phase in the crystalization process. That is, in the crystalization process, the phase transition for the crystalization is undergone from an isotropic fluid state to a chiral nematic (N*) phase to a chiral smectic C (SmC*) phase. Of the liquid crystals transiting phases from the chiral nematic phase to the chiral smectic C phase, there is a half-V type liquid crystal having a monostable characteristic.
In the half-V type liquid crystal, the optical axis is positioned in a parallel direction with a rubbing direction of the orientation film when any electric potential is not applied, as shown in FIG. 1, and, when a positive voltage (+) is applied, the long axis of the liquid crystal is tilted up to 45 degrees at maximum in correspondence with a level of the electric potential applied. In FIG. 1, Vsat denotes a saturation voltage which produces a maximum tilt for the liquid crystal.
Further, with an application of a negative voltage, the long axis of the liquid crystal is aligned in the same direction as when no potential is applied. The light transmittance of such a liquid crystal has a relationship as shown in FIG. 2, that is, a monostable characteristic with respect to an applied voltage. The liquid crystal having such a characteristic has several names, but is generally referred to as a half-V type liquid crystal, particularly considering the applied potential (V)-to-light transmittance (T) characteristic.
In particular, such a liquid crystal enables alternative current driving and analog gray scale displays for easy full-color implementations, and enables pulse driving due to the monostable characteristic, to thereby have an advantage of providing a display performance close to the existing CRT display.
However, the FLCD of the bookshelf structure which is manufactured with the conventional cooling has layer contractions occurring during the increase of a tilt angle of liquid crystal molecules when in the phase transition from the chiral nematic phase to the chiral smectic C phase in a process of the crystalization of liquid crystal, causing a problem of forming non-uniform domains in an alignment process of liquid crystal for compensating the layer contraction so that bright differences are brought out in a display state.